1. FIELD OF THE INVENTION
The invention concerns a device for detecting gases, especially toxic gases, by virtue of their absorption of infrared radiation.
2. DESCRIPTION OF THE PRIOR ART
Very many gases, including hydrocarbons, carbon oxides, water . . . have absorption bands in the infrared (this does not apply to monoatomic gases or to symmetrical diatomic gases).
For this reason it has already been proposed to use this phenomenon to detect or even to quantify the presence of a gas of this kind in an environment that in principle should not contain it.
For example, this property has been used to detect and to quantify the presence of an undesirable gas such as CO, CO.sub.2 or a saturated hydrocarbon (from methane up to butane), or even ammonia or chlorine-containing gases such as trichloroethylene, in an environment based on air that is normally breathable by humans but that has to be monitored (for example in winery cellars, glasshouses, mushroom farms, etc.).
Detectors of this kind are described in U.S. Pat. Nos. 5,026,992; 5,060,508; 5,103,096; 5,053,754; 5,163,332 and 4,709,150 and British Patent No. 1,398,977.
U.S. Pat. No. 5,026,992 proposes a detector in which the infrared radiation is emitted by an IR microsource that can be modulated electrically and embodies a small thin-film or filament resistor made from a material that can heat up and cool down very quickly. This source emits blackbody radiation. The supply of power to it is modulated so that its temperature alternates between two temperatures causing different radiation regimes.
The gas mixture flows transversely to the microsource-detector direction.
The radiation emitted, either before or after passing through the body of gas to be characterized, passes through a filter with two narrow passbands, only one of which contains an absorption band of the undesirable gas to be characterized; the other band is in theory one in which none of the gases likely to be present in the environment to be tested absorbs energy.
A PbSe (or even pyroelectric) photoconductor detector collects the filtered radiation. The signal is then processed to obtain maximal values for each of the power supply regimes of the source. The concentration of the gas to be detected is deduced from the ratio between the aforementioned maximal values.
The extreme temperatures of the source are high (250.degree. C. and 450.degree. C. for methane, for example), which in practice means that the source of radiation cannot be in communication with the gas mixture because of the risk of the source igniting the mixture. In such cases complying with safety standards entails isolating the measurement cell from the source, which imposes a penalty in terms of mass, cost and energy detected.
U.S. Pat. Nos. 5,060,508; 5,163,332; 4,709,150 and British Patent No. 1,398,977 disclose a point source (lamp, for example an incandescent filament in a transparent envelope, possibly at the focus of an ellipsoidal reflector), a detector of a kind specified in varying degrees of detail and, between the source and the detector, a light tube, the walls of which are usually highly reflective, possibly having a zig-zag shape, and running along a wall that is porous to the gases through which some of the gas mixture to be tested penetrates by diffusion. This wall prevents the ingress of any smoke or dust particles, however.
The detector is fitted with selective filters, at wavelengths depending on the gas to be detected.
The use of a point source of light combined with the use of a long light tube or with a porous tube that is presumably not highly reflective would seem to impose a high source temperature, as before, with the risk of ignition in the case of direct contact between the source and the gas to be detected if the latter is explosive. In this regard, it should be noted that the documents under discussion here are primarily concerned with detecting Co.sub.2.
It will be realized that the aforementioned structure with a sinuous light tube is complex to manufacture, with probable penalties in terms of cost, weight and overall size.
U.S. Pat. No. 5,103,096 combines the teachings (and the drawbacks) of U.S. Pat. Nos. 5,026,992 and 5,060,508.